1. Field of the Invention
The present invention relates to a method and apparatus for controlling a multi-gas sensor suitable for detection of nitrogen oxide concentration and ammonia concentration of a gas under measurement.
2. Description of the Related Art
Known gas sensors for measuring the concentration of a specific gas contained in the exhaust gas of an automobile include an NOX sensor which employs a solid electrolyte body so as to detect the NOX concentration of a gas under measurement, and an ammonia sensor which detects the ammonia concentration of the gas under measurement by making use of a change in impedance between a pair of electrodes or a change in electromotive force generated therebetween.
Also, a technique has been proposed for simultaneously measuring the NOX concentration and ammonia concentration of a gas under measurement. The proposed technique has a step in which the gas under measurement is brought into contact with an NH3 strong oxidizing catalyst so as to convert ammonia to NOX, and the overall NOX concentration is measured; and a step in which the gas under measurement is brought into contact with an NH3 weak oxidizing catalyst so as to convert a portion of ammonia to NOX, and the NOX concentration is measured. The NOX concentration and the ammonia concentration are calculated from two detection values obtained in these steps (see Patent Document 1).
Furthermore, an ammonia sensor has been proposed in which an NOX electrode, a reference electrode, a selection electrode, and an ammonia electrode are provided in a single sensor to thereby form a plurality of cells. The method of calculating ammonia concentration is changed in accordance with an electromotive force generated between the NOX electrode and the reference electrode as a result of introduction of a gas (depending on the presence of NOX), whereby the ammonia concentration is accurately measured (see Patent Document 2). Specifically, depending on the presence of NOX, an electromotive force between the ammonia electrode and the reference electrode or an electromotive force between the NOX electrode and the ammonia electrode is selectively used for detection.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2001-133447 (abstract)    [Patent Document 2] Japanese Kohyo (PCT) Patent Publication No. 2009-511859 (abstract)
3. Problems to be Solved by the Invention
However, in the case of the technique described in Patent Document 1, since measurement is performed on the basis of a difference in catalytic performance, if the catalytic performance changes due to changes in measurement conditions (temperature, flow rate, pressure, etc.), accurate measurement cannot be performed. Moreover, in the case where the apparatus of Patent Document 1 is disposed in a severe environment such as in the exhaust gas of an automobile, it is difficult to stably maintain catalytic performance over a long period of time.
Also, the technique which is described in Patent Document 2 and in which the detection electrodes to be used for measuring ammonia concentration are switched has the following problem. When the measurement environment changes, initially set conditions for switching the detection electrodes (setting of switching timing, reference output point, etc.) must be changed, and accurate measurement of ammonia concentration becomes rather difficult. Furthermore, control, i.e., switching of the detection electrodes, is complicated.
Furthermore, in the case where a plurality of gas concentrations are measured using separate sensors, the following problem occurs. Since the sensors are not provided at the same location, gas concentration, temperature distribution, etc., vary among the sensors, and measurement delay occurs in certain of such sensors. The variation and delay may adversely affect the measurement.
Moreover, oxygen concentration may influence the measurement of ammonia concentration using an ammonia sensor. In particular, conventional ammonia sensors encounter difficulty due to the influence of oxygen concentration in measuring ammonia concentration.